Multilayer ohmic contacts have been fabricated on Group III-V semiconductor materials of n-type and p-type conductivity. A representative survey of contacts and contact fabrication technology in this area is disclosed in Solid State Electronics, Vol. 18, p. 541 (1975).
With respect to contacts on p-type material, particular difficulty has been encountered in obtaining a reliable ohmic contact suited for use on small contact area optoelectronic devices. Of those processes achieving a modicum of success in simply contacting p-type material, most have employed a combination of layers including gold and zinc. Zinc is utilized because it is usually incorporated as an acceptor in the epitaxial layers of the semiconductor material. However, zinc is known to have a high vapor pressure which results in evaporation of zinc during heat treatment of the contact.
U.S. Pat. No. 3,850,688 issued to I. Halt on Nov. 26, 1974, discloses a method or fabricating a multilayer contact which is designed to suppress the evaporation of zinc during a heat treatment step required for effecting the diffusion of zinc into the p-type semiconductor material. The method involves the sequential deposition of a thick layer of gold (500-5000 angstroms) followed by a thick layer of gold-zinc alloy (1000-10,000 angstroms) and a thick second layer of gold (500-5000angstroms).
A number of other relatively thick gold and zinc multilayer contacts have been fabricated for p-type Group III-V semiconductor material. See, for example, Solid-State Electronics, Vol. 24, p. 69 (1981); J. Vac. Sci. Technol., 19(3), p. 623 (1981); J. Electrochem. Soc., Vol. 124, No. 2, p. 317 (1977); J. of Appl. Phys., Vol. 46, No. 1, p. 452 (1975); and U.S. Pat. No. 3,975,555 issued to I. Ladany et al. on Aug. 17, 1976.
In all of the references mentioned above, each ohmic contact is relatively thick and exhibits a moderate specific contact resistance (.sup..about. 10.sup.4 .OMEGA.cm.sup.2).